TODO: progress callback without doubles

 1.3 struct lifreq

 1.4 signal-based resolver timeouts

 1.5 get rid of PATH_MAX

 1.6 progress callback without doubles

 2. libcurl - multi interface

 2.1 More non-blocking

 we need libssh2 to properly tell us when we pass in a too small buffer and

 its current API (as of libssh2 1.2.7) doesn't.

1.6 progress callback without doubles

 The progress callback was introduced way back in the days and the choice to

 use doubles in the arguments was possibly good at the time. Today the doubles

 only confuse users and make the amounts less precise. We should introduce

 another progress callback option that take precedence over the old one and

 have both co-exist for a forseeable time until we can remove the double-using

 one.

2. libcurl - multi interface

2.1 More non-blocking